Anab InitioComputational Molecular Orbital Study of Radical, Protonated Radical (Radical Cation), and Carbocation Species That Have Been Proposed in Mechanisms for the Transfer Process in the Enzyme−Coenzyme B₁₂-Catalyzed Dehydration of 1,2-Dihydroxyethane

Abstract

The mechanism of action of the enzyme diol dehydrase has variously been suggested to involve a radical, a protonated radical (radical cation), or a carbocation in the apparent transfer of HO from one carbon atom to the other to give acetaldehyde and water. This adenosylcobalamin-requiring B 12 enzyme catalyzes dehydration of 1,2dihydroxyethane. Its active site is buried within a hydrophobic cavity. Each of the three possibilities are examined by ab initio molecular orbital calculations. Total molecular energies have been calculated with full geometry optimization at the MP2(FC)/6-31G* level and, in addition, single point energies using the MP2(FC)/6-311++G** basis set. Vibrational frequencies were also calculated. Transfer of an HO group within a HOCH-CH 2 OH • radical (postulated to be formed by the initial reaction of the diol with the deoxyadenosyl radical via a bridge structure) was ruled out because the activation energy is much too high in relation to the observed rate constant for the enzyme reaction. A carbocation mechanism also presents a problem, quite apart from the necessity of postulating some acceptor for the electron from the HOCH-CH 2 OH • radical. In principle acetaldehyde could be formed via the 2,2-dihydroxyeth-1-yl cation which was found to undergo a spontaneous 1,2-hydride ion shift, giving protonated acetic acid. But, although the 1,2-dihydroxyethyl cation (otherwise protonated glycolaldehyde) is well established as a stable species, the intermediary bridge structure could not be found. The radical cation HOCH-CH 2 OH 2 •+ , formed by proton transfer from an active-site group, was found to be inherently unstable, transforming without activation into a stable hydrogen-bonded hydrate of the anti-vinyl alcohol radical cation, H 2 O‚‚‚HOCH-CH 2 •+ . Deprotonation and H-atom transfer (from AdCH 3 ) were then found to give stable hydrogen-bonded hydrates of the formylmethyl radical, protonated acetaldehyde, and acetaldehyde itself. The ultimate formation of acetaldehyde and water can be attributed either to the dissociation of acetaldehyde hydrate or the prior dissociation of the formylmethyl radical hydrate followed by the H-atom transfer step. Because H 2 O is already formed as a discrete entity in the initial protonation step and no transfer of a bonded HO, H 2 O, or H 2 O + group from one carbon atom to the other actually occurs, this series of reactions may be termed a "predissociation" mechanism. The overall proton transfer and the formation of water are complicated interconnected processes. A consideration of whether or not the cobalt participates in one of the later reaction steps is underway.

The Dehydration Step in the Enzyme-Coenzyme-B12 Catalyzed Diol Dehydrase Reaction of 1,2-Dihydroxyethane Utilizing a Hydrogen-Bonded Carboxylic Acid Group as an Additional Cofactor: A Computational Study

“…Besides radical species, protonated radicals (radical cations) and carbocations have also been considered as reaction intermediates. 1c,g, To judge the feasibilities of various pathways that have been proposed, ab initio molecular orbital calculations with full geometry optimization using the MP2(FC)/6-31G* level and single-point energy determinations at the MP2(FC)/6-311++G** level were carried out . In agreement with earlier MNDO SCF-MO calculations, the protonated 1,2-dihydroxyeth-1-yl radical (H 2 O + CH 2 −ĊHOH) was found to be very unstable, breaking down spontaneously to give the vinyl alcohol radical cation (H 2 C−CHOH ⎤•+ ) and water.…”

Roaming-like Mechanism for Dehydration of Diol Radicals

“…Dehydration mechanisms are varied depending on the conditions. The aqueous-phase reactions are typically explained via acid/base-catalysis, ,− although the uncatalyzed dehydration is also known . The acid-catalyzed pinacol rearrangement initiated by protonation of a hydroxyl group, for instance, converts 1,2-diols to aldehydes or ketones .…”

Proton Transfer from Histidine 244 May Facilitate the 1,2 Rearrangement Reaction in Coenzyme B12-dependent Methylmalonyl-CoA Mutase

“…Methylmalonyl-CoA mutase catalyzes the 1,2 rearrangement of methylmalonyl-CoA to succinyl-CoA and is the only coenzyme B 12 -dependent enzyme that is present in both microbial and animal kingdoms (1,2). The role of the B 12 cofactor or adenosylcobalamin (AdoCbl) 1 is to function as a free radical reservoir responsible for the controlled generation of a substrate radical at the initiation of the reaction cycle (3).…”

“…In the next step, an interchange of the carbonyl-CoA group and a hydrogen atom occurs between vicinal carbons. The precise mechanism by which the substrate radical isomerizes to the product radical is unresolved, with pathways involving free radical intermediates (3), fragmentation products (11), carbocations (12), carbanions (13), and organocobalt adducts (14,15), each having been proposed (Scheme II). The final step involves reabstraction of a hydrogen atom from deoxyadenosine to generate product and the intact cofactor, AdoCbl.…”